In the previously known applications of automatic information handling which involve the transmission of messages between remote stations the costs are largely determined by the complexity of the members of the system controlling the transmission, and likewise of the programs or microprograms associated with their operation. To a large extent, this complexity results from the structure of the connecting networks, and from the rules for linking the messages, constituting the procedure which has been agreed between the various stations.
In current practice the connecting channels are of the type using metallic lines or radio, which convey series of modulated signals, and the messages are series of characters, generally selected in a binary code.
A distinction is conventionally made between at least two classes of message, the arrangement of which, at the level of the characters, is precisely determined within the framework of a given procedure: firstly the class of text messages, which convey the data, and secondly, the class of service messages.
A text message comprises on the one hand functional characters which identify its nature and on the other hand a series of characters constituting a text, which represent the actual data which the station transmitting the message is sending to one or more receiving stations.
The service messages ensure the coherence of the exchanges, by assuming the functions of selection, of interrogation, of positive acknowledgement of reception (transmission recognized as correct) or negative acknowledgement (error in transmission recognized), of indicating the end of transmission any any other functions considered advisable according to the nature of the network.
In the present networks, and their associated procedures, a large number of different service messages are provided. In proportion, the time devoted to occupying the connecting channel for service messages is high in comparison with the time devoted to the text messages, which considerably reduces the effective output of information in comparison with the nominal capacity of the connecting channel used.
On the other hand, the complexity of the means providing control of the network increases greatly with the number of service messages and the number of combinations which result in time.
Finally, the multiplication of the cases of error with the number of combinations of messages means that complex and extensive error avoiding, error detection and error correcting means have to aid these controlling members to ensure the continuity of the service.
Furthermore, if exchange of messages must be able to take place between a number of stations greater than two, a main or master control device is generally provided to centralize all the messages transmitted by the various stations and to process them with a view to their retransmission to the ultimate destination station or stations. This central control device obviously increases in complexity with the numbers of stations in the system. Moreover, when it is desired to evolve this system, by eliminating, or more often by adding stations, it becomes necessary to make corresponding modifications to the central control device. Likewise, it is obvious that a disturbance or breakdown affecting this central control device has repercussions on the operation of the whole of the system.